Among the polyethylenes, the PE-UHMW types occupy a special position. These are understood as meaning linear polyethylenes which have an extremely high melt viscosity and are obtained by the low pressure process. They are characterized by a limiting viscosity number of 1000 to 5000 ml/g.
Ultra-high molecular weight polyethylene is characterized by a number of physical parameters which open up various possible uses to it. Its high wear resistance, its low coefficient of friction with respect to other materials, and its excellent toughness are worthy of being singled out. Moreover, it is remarkably resistant to numerous chemicals.
On the basis of its favorable tribological and chemical properties and its high toughness, PE-UHMW has found acceptance as a versatile material in very widely varying fields of use. Examples are the textile industry, mechanical engineering, the chemical industry, and mining.
A significant proportion of the PE-UHMW obtained as a powder during its synthesis is sintered by various processes to semi-finished products, from which shaped articles are produced by cutting, for very widely varying fields of use. However, there are often difficulties in producing shaped articles from PE-UHMW by the methods customary for thermoplastics. Ultra-high molecular weight polyethylene tends to form a skin to varying degrees (delamination) when processed by the conventional injection molding technique. The degree depends on the process parameters applied, particularly the length of the mold sections which are flat in the direction of flow. Flaky layers form on the surface of the molding and can easily be peeled off from the core material. This phenomenon leads to increased wear of the shaped article produced from PE-UHMW, which contrasts with the excellent wear resistance of workpieces produced by pressing processes. Various measures have been taken to eliminate this deficiency. They comprise, in particular, a special design of the injection molding machines or provision of modified ultra-high molecular weight polyethylenes which are suitable for injection molding.
According to DE-C-2,600,923, to form powder particles, molten PE-UHMW is injected at 140.degree. to 300.degree. C. and at a shear rate of 50,000 to 1 million second.sup.-1, measured at the crosspiece of the injection nozzle, into a die cavity. The volume of the cavity is 1.5 to 3.0 times greater than the volume of the polyethylene injected, and the volume of the hollow mold is then reduced to less than 1.5 times that of the polyethylene injected.
FR-A-2,503,625 relates to a further development of the process described above. It comprises injecting PE-UHMW, heated to 170.degree. to 240.degree. C., into a mold cavity which increases its volume by at least 40% during this operation and decreases in size when the injection has ended, while the material in the mold is still in the molten state.
The two foregoing processes require the use of injection molding machines which are equipped with special molds, in particular those having variable hollow mold volumes. The usual injection molding units are unsuitable for carrying out these processes.
Instead of employing particular processing machines, special polymers have also been developed for the production of shaped articles from PE-UHMW by injection molding. U.S. Pat. No. 4,786,687 relates to ultra-high molecular weight polyethylenes which are suitable for processing by injection molding and are obtained in a two-stage process. In a first stage, ethylene is polymerized in the presence of a solid catalyst, comprising magnesium, titanium, and/or vanadium and an organometallic compound, in the absence of hydrogen. Ethylene is then polymerized in the presence of hydrogen in the same reactor in a second stage.
A polyethylene composition which comprises PE-UHMW and is suitable for injection molding is likewise obtained in a multi-stage process by a method described in EP-B-02 74 536. Ethylene is polymerized in at least one polymerization stage in the absence of hydrogen and in another polymerization stage in the presence of hydrogen. A catalyst of the Ziegler type, which comprises a highly active catalyst component containing magnesium, titanium, and halogen as essential constituents and an organoaluminum compound as a further catalyst component, is present in each of the individual reaction stages.
Finally, the patent literature also contains references to PE-UHMW compositions which are moldable and are prepared by mixing ultra-high molecular weight polyethylene with polyethylene or other polyolefins of lower molecular weight. According to the published Japanese Patent Application 177036/1982, such molding compositions comprise 100 parts of polyethylene having a molecular weight above 10.sup.6 and 10 to 60 parts of polyethylene having a molecular weight of 5000 to 20,000. The published Japanese Application 126446/1984 describes mixtures of 50 to 95 parts of ultra-high molecular weight polyethylene and 50 to 5 parts of an all-purpose polyolefin. None of the publications deals with processing of these compositions by injection molding.